Photohydrogenation of acetylenes in water-oil two-phase systems: Application of novel metal colloids and mechanistic aspects of the process

Photohydrogenation of phenylacetylene and methylphenylacetylene is accomplished in a water-cyclohexane system using tris(bipyndme)ruthenium(II), Ru(bpy)₃²⁺, as a photosensitizer, N,N′-dialkyl-4,4′-bipyridinium (viologen), C_nV²⁺, as a charge relay, and Na₂EDTA as a sacrificial electron donor in the photosystem and a Pt or Pd colloid stabilized in the organic phase as a hydrogenation catalyst. The photogenerated bipyridinium radical cations undergo induced disproportionation in the water-oil two-phase system, and the two-electron charge relay C_nV is the active photoproduct that charges the metal colloid and generates metal-bound H atoms that are active in the hydrogenation of the substrate. The Pt and Pd colloids differ in their effectiveness in the generation of metal-bound H atoms. While Pt is a superior catalyst in this function, Pd is superior to Pt in the activation of the substrate toward hydrogenation. Use of a mixture of Pt and Pd colloids in a water-oil two-phase system shows a synergetic catalytic activity in the photohydrogenation of the acetylenic substrates.